In transmission devices known from practice and respectively designed as automatic transmissions constructed with frictional shift elements for producing various gear ratios, the gear changes required are carried out as controlled change-under-load shifts during which, in each case, at least one functional shift element of the transmission device is disengaged from the flow of force of a vehicle drivetrain and, correspondingly, at least one frictional shift element is engaged in the force flow. To keep to predefined shift times, which in each case correspond to the time interval between disengagement of the currently engaged gear ratio engaged to engagement of the target gear ratio, during the gear change so-termed nominal contact times of the frictional shift elements involved in the respective shift are specified.
From the nominal contact times a gradient of a nominal rotation speed of a transmission input speed or a nominal speed value equivalent thereto is determined, which is compared with a gradient of an actual speed value equivalent thereto, which can be determined from actual contact times of the shift elements. Depending on the deviation determined in each case between the gradient of the nominal speed and the gradient of the actual speed, and as a function of the operating situation of a vehicle's drivetrain at the time, an actuation pressure of a shift element to be engaged when engaging the required target gear in the force flow of the vehicle's drivetrain, or an actuation pressure of a shift element to be disengaged in order to disengage the gear engaged at the time from the force flow, is adjusted. In addition, the load transfer to be carried out during the shift, namely from the shift element being disengaged to the shift element being engaged, and the gear interval to be bridged during the nominal contact time and the speed gradient necessary for this, are realized by a pressure adjustment in the area of the shift element to be engaged or to be disengaged.
Since frictional shift elements in the disengaged operating condition produce a drag torque that has an adverse effect on the efficiency of an automatic transmission, transmission devices are increasingly made at least in part with interlocking instead of frictional shift elements, since interlocking shift elements essentially cause no drag torque and, compared with frictional shift elements, can transmit substantially higher torques while at the same time taking up less structural space.
The procedure described above cannot be carried out with transmission devices which are made at least in part with interlocking shift elements, since in contrast to frictional shift elements, in the case of interlocking shift elements the transmission capacity cannot be varied continuously by a pressure adaptation. Thus, it is not possible by means of a pure pressure regulation of a shift element either to produce reproducible shift times and the adaptation of the space gradients needed for this, or to establish a specified contact time and carry out the load transfer within a predefined shift time in shift operations during which a load transfer takes place between interlocking and frictional shift elements.
For example, if in a transmission device a thrust upshift is called for, to carry out which an interlock-type shift element has to be disengaged from the force flow of a vehicle's drivetrain and a frictional shift element has to be engaged in the force flow, no pressure regulation can be carried out in the area of the interlocking shift element. Furthermore, before being disengaged, the interlocking shift element has to be brought to an at least approximately load-free condition as a preliminary to the disengagement process, and to do this the thrust torque in the area of the interlocked connection or the interlock-type shift element must be reduced.
Moreover, for example in the case of a thrust downshift to carry out which a frictional shift element has to be disengaged from the force flow of the vehicle's drivetrain and an interlock-type shift element has to be engaged in the force flow, a transmission input speed or a transmission speed equivalent thereto has to be changed from the level of the synchronous speed of the actual gear engaged at the time to the level of the synchronous speed of the target gear, but this cannot be done by pressure regulation in the area of the frictional shift element.
Thus, the requirements for shift operations during which a load transfer has to take place between an interlock-type and a frictional shift element cannot be met in a satisfactory manner and with reproducible shift times.